In recent years, a data writing technique known as “shingled write” or “shingled write magnetic recording (SMR)” has been developed as a technique achieving a high storage capacity, in disk storage apparatuses a representative example of which is the hard disk drive (HDD). Such a data writing method will be hereinafter referred to as “shingled write method.”
The singled write method is a data writing method in which data is written in units of tracks provided on a disk, writing data in not only each track, but also partially overlapping the current track with previously written adjacent track. While data is being written in a track by the shingled write method, a phenomenon called “drift off write” may occur. That is, the head may deviate from the centerline of the track in which the data is being written. The term “drift off” is almost identical in sense to “track off.”
If drift off write occurs during the shingled write, the data will be written in a track already holding other data. The other data may inevitably be degraded. If the other data is degraded, it may be subjected to a so-called “refresh process,” also known as a “rewrite process.” Once the data in the track has been erased, however, the refresh process can no longer be performed to read and save the data.